Method and system for avoiding bad frequency subsets in a frequency hopping cordless telephone system

ABSTRACT

A method and system are disclosed for avoiding bad frequency subsets in a frequency hopping cordless telephone system. A base station communicates with the handsets using frequencies selected from active frequency subsets. Bad frequency subsets are avoided by monitoring the air interface between the base station and handsets for errors in active frequency subsets. Errors in each active frequency subset are counted during a short-term interval using an associated short-term error counter and during a long-term interval using an associated long-term error counter. After each short-term interval, it is evaluated whether any of the short-term error counters has a value that is greater than a defined threshold. After each long-term interval, it is evaluated whether any of the long-term error counters has a value greater than an error count for a blocked frequency subset. A blocked frequency subset is then substituted for an active frequency subset if an associated short-term error counter is greater than the defined threshold or if a long-term error counter is greater than the error count for a blocked frequency subset.

This application is related to U.S. patent application Ser. No.09/113,539, filed Jul. 10, 1998, entitled “Method and System for TableImplemented Frequency Selection in a Frequency Hopping CordlessTelephone System”, pending, and U.S. patent application Ser. No.09/113,415, filed Jul. 10,1998, entitled “Method and System for ShiftingFrequency Subsets to Avoid Base Station Interference in a FrequencyHopping Cordless Telephone System”, pending, the disclosures of whichare incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of cordlesscommunication systems and, more particularly, to a method and system foravoiding bad frequency subsets in a frequency hopping cordless telephonesystem.

BACKGROUND OF THE INVENTION

Cordless or wireless communications systems are widely used to provideusers with mobile communications. In general, the term cordless canrefer to any form of air wave transmission using a set of radiofrequencies. Conventional implementations of cordless systems, forexample, include both public cordless systems and in-building cordlesssystems. In public systems, there are numerous service providers thatallow users to make and receive calls virtually anywhere within aservice area Such service providers offer solutions based on a number ofdifferent technologies and standards. Typically, the service providershave purchased a license from the federal government (i.e., FederalCommunications Commission) to use a specific portion of the radiospectrum within specific markets.

In contrast to public systems, in-building cordless systems can avoidthe costs of radio spectrum licenses by using unlicensed radiofrequencies. In-building systems typically have a common configurationor topography in that there is a radio exchange that is adjunct to orintegrated with a private branch exchange (PBX). Base stations (or fixedparts) are equipped with radio antennas that connect to the radioexchange. The base stations also transmit radio signals to and receiveradio signals from cordless handsets (portable parts) within a limitedrange.

With respect to unlicensed radio frequencies, cordless systems often usethe ISM (Industrial, Scientific and Medical) band. In the United States,ISM based devices are regulated by and must follow FederalCommunications Commission (FCC) guidelines. In general, FCC guidelinesimplement restrictions on the use of frequencies within the ISM band.For example, devices are allowed to communicate at a particularfrequency only with a defined bandwidth for a defined period of time andwith a defined signal power level. Since the ISM band is unlicensed, itis used by many vendors for various types of cordless devices (e.g.,medical monitoring devices, wireless LANs, printers, speakers, securitysystems and in-building cordless systems). Consequently, radio frequency(RF) interference can be a significant problem with using the ISM band.

For a cordless telephone system using the ISM band, the FCC restrictionsproduce a need to implement a frequency hopping scheme to ensure thatthe cordless system does not violate restrictions on the use offrequencies within the ISM band. Frequency hopping can achieve this byallowing base stations and handsets to move in sync from frequency tofrequency in the time domain. Further, when implementing such afrequency hopping scheme, there is a need to implement a scheme foravoiding bad channels or frequencies due to RF interference and otherproblems.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method and system foravoiding bad frequencies in a frequency hopping cordless telephonesystem are disclosed that provide advantages over conventional cordlesstelephone systems.

According to one aspect of the present invention, a base stationcommunicates with handsets using frequencies selected from activefrequency subsets. Bad frequency subsets are avoided by monitoring theair interface between the base station and handsets for errors in activefrequency subsets. Errors in each active frequency subset are countedduring a short-term interval using an associated short-term errorcounter and during a long-term interval using an associated long-termerror counter. After each short-term interval, it is evaluated whetherany of the short-term error counters has a value that is greater than adefined threshold. After each long-term interval, it is evaluatedwhether any of the long-term error counters has a value greater than theerror count for a blocked frequency subset. A blocked frequency subsetis then substituted for an active frequency subset if an associatedshort-term error counter is greater than the defined threshold or if along-term error counter is greater than the error count for a blockedfrequency subset.

A technical advantage of the present invention s the abilityautomatically to block bad frequency subsets and to adapt to changinginterference by substituting blocked subsets for active frequencysubsets.

Other technical advantages should be apparent to one of ordinary skillin the art in view of the specification, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 is a block diagram of one embodiment of a frequency hoppingcordless telephone system;

FIG. 2 is a block diagram of one embodiment of frame frequencies for afrequency hopping cordless telephone system;

FIG. 3 is a diagram of one embodiment of subdividing the ISM band for afrequency hopping cordless telephone system;

FIG. 4 is a state diagram of one embodiment of a method for avoiding badfrequency subsets in a frequency hopping cordless telephone system;

FIG. 5 is a diagram of one embodiment of shifting frequencies to avoidbase station interference in a frequency hopping cordless telephonesystem; and

FIG. 6 is a state diagram of one embodiment of a method for shiftingfrequencies to avoid base station interference in a frequency hoppingcordless telephone system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of one embodiment of a frequency hoppingcordless telephone system, indicated generally at 10. System 10comprises one or more base stations 12, each which can also be referredto as a fixed part (FP). Each base station 12 can support communicationwith a plurality of handsets 14 and handsets 16 using radio frequencies.The interface between base station 12 and handsets 14 and 16 can bereferred to as the air interface. Handsets 14 and handsets 16 car. alsobe referred to as portable parts (PP).

In operation, base station 12 can support a defined total number ofhandsets 14 and 16. For example, in one implementation, base station 12can support a total of eight handsets, either idle locked or activelocked. Of the total number of handsets, a given number “M” can beactive locked handsets 16. For example, base station 12 could support upto four active locked handsets 16 from the eight total handsets. Of theremaining handsets, base station 12 can support a given number “N” ofidle locked handsets 14. For example, “N” can be less than or equal tothe difference between the total number of supported handsets (e.g., 8)and the number “M” of active locked handsets 16 (e.g., 0-4). Idle lockedhandsets 14 are handsets that are currently inactive but are in contactwith and in sync with base station 12.

Base station 12 can communicate with handsets 14 and handsets 16 using atime division multiplexed (TDM) frame-based communication protocol. Forexample, each frame can be ten milliseconds (10 ms) in duration and caninclude transmit and receive channels for communication and controldata. One protocol used with digital cordless telephone systems is theDigital Enhanced Cordless Telecommunications (DECT) protocol, which isthe pan-European standard for digital cordless systems and supports upto six locked handsets 16 (i.e. , M=6). There are, of course, otherprotocols used for communicating across the air interface between basestation 12 and handsets 14 and handsets 16. For example, the DECTprotocol can be modified to support up to four locked handsets 16 (i.e.,M=4) each with enhanced communication features due to higher data rates.

In the embodiment of FIG. 1, system 10 uses an ISM band of radiofrequencies for supporting communication between base station 12 andhandsets 14 and 16. For example, system 10 can use the ISM bandextending from 2.4 GHz to 2.4835 GHz. An advantage of using the ISM bandis that it is unlicensed and does not require a license fee for use.However, in order to operate within FCC or other government regulations,system 10 implements a frequency hopping scheme. This allows system 10to support robust cordless communications in the ISM band whileoperating within regulation guidelines. Under the frequency hoppingscheme, base station 12 and handsets 14 and 16 move in the time domainfrom frequency to frequency. Because of the changing frequency, handsets14 and 16 are initially in an unlocked stare when entering an areaserviced by base station 12. Handsets 14 and 16 can then “listen” at aspecific radio frequency to attempt to lock on to base station 12. Whenbase station 12 hops to that frequency specific frequency, handsets 14and 16 can identify and receive control data transmitted by base station12. This allows handsets 14 and 16 to lock with base station 12 and syncwith the frequency hopping scheme.

FIG. 2 is a block diagram of one embodiment of frame frequencies for afrequency hopping cordless telephone system. As shown, a framestructure, indicated generally at 20, comprises a plurality of frames 22each having a frame length 24. Each frame 22 follows immediately afterthe previous frame 22 in the time domain. In the embodiment of FIG. 2, adifferent frequency (F₁, F₂, F₃ . . . F_(N), Fe_(n+1), . . . ) isassociated with each frame 22 and is used during that frame 22 forcommunication across the air interface between base station 12 andhandsets 14 and 16. This change from frequency to frequency is handledby the frequency hopping scheme implemented by base station 12 andhandsets 14 and 16. During the duration of a given frame 22, basestation 12 and handsets 14 and 16 communicate using the selectedfrequency for that frame 22. When the next frame 22 begins, base station12 and handsets 14 and 16 communicate using a new selected frequency. Inone embodiment, frame length 24 is ten milliseconds, thus the frequencybeing used changes every ten milliseconds.

FIG. 3 is a diagram of one embodiment of subdividing the ISM band for afrequency hopping cordless telephone system. The ISM band used in thisembodiment extends from 2.4 GHz to 2.4835 GHz. As mentioned, the FCCdefines requirements for use of frequencies within the ISM band. Forexample, within a 30 second period, the regulations limit the maximumlength of time that a system can use one frequency to 0.4 seconds. Thus,the total available frequencies needs to include seventy-five or morefrequencies. In the embodiment of FIG. 3, this range is divided intotwelve subsets 30, and each subset 30 is divided into eight channels 32.Each channel 32 is hen associated with one of ninety-six frequencies 34defined within and equally subdividing the ISM band. Frequencies 34 thenprovide a set of frequencies from which the frequency hopping scheme canselect for each frame 22.

The frequency hopping scheme, in addition to selecting frequencies, alsoneeds to implement a scheme for avoiding bad frequency subsets. Forexample, a PCS microwave tower may interfere with frequencies in the ISMband in a particular region. Thus, cordless telephone system 10 wouldnot want to use those frequencies. One way to avoid such bad frequencysubsets is to block their selection. By dividing the ISM band intoninety-six frequencies, the embodiment of FIG. 3 provides sufficientfrequencies to allow bad frequencies to be blocked while keeping thenumber of available frequencies above the seventy-five frequencythreshold. For example, there is freedom to avoid using the frequencieswithin two subsets 30 without dropping below the seventy-five frequencythreshold.

Within a cordless telephone system, a frequency hopping scheme canaddress a number of implementation problems. For example, the frequencyhopping scheme should be consistent across base stations yet attempt toensure that adjacent base stations do not select the same frequenciesand interfere with one another. This means that the frequency selectionprocess needs to be both predictable (so handsets can lock with any basestation, and variable (so base stations operate at differentfrequencies). Also, the frequency hopping scheme should attempt to avoidselecting and using bad frequency subsets due to interference or otherproblems. Further, the frequency hopping scheme should react tointerference by separating the frequencies selected by a base stationfrom possible interference by other base stations.

FIG. 4 is a state diagram of one embodiment of a method for avoiding badfrequency subsets in a frequency hopping cordless telephone system. Themethod of FIG. 4 can be implemented by a base station to enhanceselection of frequencies for the air interface quality by avoiding badfrequency subsets. As mentioned above with respect to FIG. 3, oneembodiment of a frequency hopping cordless telephone system definestwelve different subsets for grouping channels within the ISM band. Themethod of FIG. 4 operates to select for use the current best ten out ofthe twelve available subsets and to block the remaining two subsets.Thus, in this embodiment, the system uses ten subsets, except duringsubset substitution as is described below with respect to FIGS. 5 and 6.

In general, the method of FIG. 4 involves a number of steps in theselection of which subsets to block and uses two error countersassociates with each subset. The first error counters for each subsetare compared and cleared every second. The second error counters arechecked and reset every five seconds. Consequently, the first errorcounters can be referred to as the short-term error counters, and thesecond error counters can be referred to as the long-term error countersin this embodiment, both the short-term and long-term error counters areincremented if any of he following conditions are met: (a) bad packetdata, indicated by a bad synchronization word; or (b) bad cyclicredundancy code (CRC). Substitution of currently blocked subset for acurrently active subset is then performed if either of the followingconditions are met: (a) in a one second period any of the short-termerror counters for an active subset has a count that is greater thannine; or (b) in a five second period any of the long-term error countersfor an active subset has a count greater than the count for one of theblocked subsets. Further, in this scheme, every five seconds the errorcount for each of the blocked subsets is reduced by 2.5%. This reductionis repeated up to six times until the error count is equal to 85% of theoriginal value (i.e., 100%−6×2.5%=85%).

Specifically, the embodiment of FIG. 4 implements the method using astate machine having five states. Initially, the method is in an idlestate 40 in which the method for avoiding bad frequency subsets isinactive. When frequency selection is initiated, the method then movesto an init state 42. in init state 42. the short-term and long-termerror counters and data relating to blocked subsets are initialized.Each short-term counter can be an eight bit counter that is clearedevery second, while each long-term counter can be a sixteen bit counterthat is cleared every five seconds, As mentioned, there is a pair ofcounters associated with each subset. The data relating to the blockedsubsets stores information about each of the blocked subsets and theerror count. This data can be updated when there is a new subset to beblocked. Init state 42 is used once, and the method moves to a countingstate 44.

There are two components to counting state 44. The first is themonitoring of the air interface quality for errors in active subsets,and the second is the timing of one and five second intervals forcounting errors in active subsets. The third component is an evaluationwhether any of the short-term error counters is greater than a definedthreshold (e.g. a count of nine). The method moves directly to achanging state 46 if any of the short-term error counters is greaterthan the threshold, otherwise the method remains in counting state 44until five seconds have passed. At the end of five seconds, the methodmoves to an evaluation state 48 and evaluates whether there is a blockedsubset to be substituted for an active subset based upon the long-termerror counters. In evaluation state 48, the long-term error counters arecompared. If the count for an active subset is greater than that for ablocked subset, then the data relating to blocked subsets is updated toreflect a substitution, and the method moves to changing state 46.

In changing state 46, a blocked subset is made active and an activesubset is blocked. The active subset to be blocked is either one thathad a short-term error count greater than the threshold (from countingstate 44) or one that had a long-term error count greater than one ofthe blocked subsets (from evaluation state 48). Changing state 46results in the substitution of the blocked subset for the active subsetand returns to counting state 44. This substitution process needs to beseamless to avoid loss of synchronization. Further, during thesubstitution process, a scheme can be implemented for shiftingfrequencies to avoid base station interference.

FIG. 5 is a diagram of one embodiment of shifting frequencies to avoidbase station interference in a frequency hopping cordless telephonesystem. Because base stations within a cordless telephone system arelikely to implement the same frequency hopping and bad frequencyavoidance schemes, collisions can occur and persist. The subset orfrequency shifting of FIG. 5 avoids these collisions which can occurwhen two or more base stations use the same channel at the same time. Asmentioned above, in one embodiment, the base station's frequency hoppingscheme cycles through ten active subsets before repeating a subset. Thesubset or frequency shifting is implemented by having the system cyclethrough eleven subsets instead of ten for a certain time when acollision occurs.

As shown in FIG. 5, the base station being shifted can implement oneadditional hop after the normal ten hops. As a result, the shiftedsystem breaks the period of the use of the subset. For example, withoutshifting, Subset 1 would appear at hop numbers 1, 11, 21 and so on. Withshifting using an additional subset hop, Subset 1 is shifted to hopnumbers 12, 23 and so on. Thus, in this scheme, Subset 1 is shifted witha rate of {fraction (1/10)}. As should be clear, using eleven subsetsfor five cycles would shift Subset 1 to fall in the middle of theposition it would otherwise occupy. This can be considered to be anoptimal shift to avoid base station interference.

This subset shifting can be implemented as part of the above method foravoiding bad frequency subsets. In particular, subset shifting can beintegrated as part of subset substitution in the changing state. Thus,both the newly active blocked subset and the to-be-blocked active subsetcan be used for a defined period of time to increase the number ofsubset hops and accomplish subset shifting.

FIG. 6 is a state diagram of one embodiment of a method for thisshifting of frequencies to avoid base station interference in afrequency hopping cordless telephone system. In this embodiment, subsetshifting is implemented as part of the subset substitution of the methodof FIG. 4. As shown an FIG. 6, subset shifting can be implemented by astate machine having an init state 50, a releasing state 52, a waitingstate 54 and a changing state 56.

During waiting state 54, the system hops using eleven subsets, andsubset shifting is taking place. In this process, information relatingto blocked subsets can be stored (e.g., as a variable), and the basestation can broadcast the information to the handsets. The handsets needto receive this broad cast information before the system hops to thesubset that is related to the information. This is applied for bothreusing a blocked subset as well as blocking the active subset. Duringinit state 50, the method determines the subset number where the subsetblocking information is updated. An algorithm is implemented such thatthe subset blocking information is updated while the subset followingthe blocked subset to be released is used. For example, Subset 10 mightbe a blocked subset that will be released. Assuming the hop sequence is8, 9, 10, 11 and so on, then the system waits until Subset 11 beforeremoving the information about Subset 10 from the subset blockinginformation. After init state 50, the method moves to releasing state52.

In releasing state 52, the method waits until the subset blockinginformation can be updated. After the blocked subset is released, themethod moves to waiting state 54, and the system hops with elevensubsets. A counter can then be initialized to allow the use of theeleven subsets for a defined period of time (e.g., fifty hops) withinwaiting state 54. As soon as the system completes the defined period oftime hopping with eleven subsets, then the active subset to-be-blockedis blocked, in changing state 56, by updating the subset blockinginformation. This method of releasing can also be implemented tomaintain synchronization. As should be understood, after the definedperiod of time (e.g., fifty hops) with eleven subsets, the subsets havebeen shifted (e.g., by five subsets) from the position they wouldotherwise occupy. As also should be understood, other implementationscan use different numbers of active subsets and hops during the definedshifting period.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade thereto without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A system for avoiding bad frequency subsets in afrequency hopping cordless telephone system, the system comprising: aplurality of handsets; and a base station communicating with thehandsets using frequencies selected from active frequency subsets, thebase station avoiding bad frequency subsets by: monitoring an airinterface between the base station and the handsets for errors in activefrequency subsets; counting errors in each active frequency subsetduring a short-term interval using an associated short-term errorcounter and during a long-term interval using an associated long-termerror counter; evaluating, after each short-term interval, whether anyof the short-term error counters has a value that is greater than adefined threshold; evaluating, after each long-term interval, whetherany of the long-term error counters has a value greater than an errorcount for a blocked frequency subset; and substituting a blockedfrequency subset for an active frequency subset if an associatedshort-term error counter is greater than the defined threshold or if along-term error counter is greater than the error count for a blockedfrequency subset.
 2. The system of claim 1, wherein the base station caninitialize the short-term error counters, the long-term error countersand data relating to blocked frequency subsets and associated errorcounts.
 3. The system of claim 1, wherein counting errors comprisesincrementing the short-term error counters and the long-term errorcounters based upon bad packet data, indicated by a bad synchronizationword, and based upon bad cyclic redundancy checks.
 4. The system ofclaim 1, wherein the short-term interval is one second, and thelong-term interval is five seconds.
 5. The system of claim 4, whereineach short-term counter is an eight bit counter that is cleared everysecond, while each long-term counter is a sixteen bit counter that iscleared every five seconds.
 6. The system of claim 5, whereinsubstituting is accomplished by updating the data storing informationabout the blocked frequency subsets to reflect a substitution.
 7. Thesystem of claim 1, wherein evaluating whether any of the long-term errorcounters has a value greater than the error count for a blockedfrequency subset comprises reducing the error count for a blockedfrequency subset after each long-term interval for a defined number oflong-term intervals.
 8. The system of claim 1, wherein there are twelvefrequency subsets of which ten are active and two are blocked.
 9. Thesystem of claim 8, wherein the twelve frequency subsets each compriseeight frequencies from a frequency band divided into ninety-sixfrequencies.
 10. The system of claim 9, wherein the frequency band is anISM band from 2.4 GHz to 2.4835 GHz.
 11. A method for avoiding badfrequency subsets in a frequency hopping cordless telephone system, themethod comprising: monitoring an air interface between a base stationand handsets for errors in active frequency subsets; counting errors ineach active frequency subset during a short-term interval using anassociated short-term error counter and during a long-term intervalusing an associated long-term error counter; evaluating, after eachshort-term interval, whether any of the short-term error counters has avalue that is greater than a defined threshold; evaluating, after eachlong-term interval, whether any of the long-term error counters has avalue greater than an error count for a blocked frequency subset; andsubstituting a blocked frequency subset for an active frequency subsetif an associated short-term error counter is greater than the definedthreshold or if a long-term error counter is greater than the errorcount for a blocked frequency subset.
 12. The method of claim 11,further comprising initializing the short-term error counters, thelong-term error counters and data relating to blocked frequency subsetsand associated error counts.
 13. The method of claim 11, whereincounting errors comprises incrementing the short-term error counters andthe long-term error counters based upon bad packet data, indicated by abad synchronization word, and based upon bad cyclic redundancy checks.14. The method of claim 11, wherein the short-term interval is onesecond, and the long-term interval is five seconds.
 15. The method ofclaim 14, wherein each short-term counter is an eight bit counter thatis cleared every second, while each long-term counter is a sixteen bitcounter that is cleared every five seconds.
 16. The method of claim 11,wherein substituting is accomplished by updating the data storinginformation about the blocked frequency subsets to reflect asubstitution.
 17. The method of claim 11, wherein evaluating whether anyof the long-term error counters has a value greater than the error countfor a blocked frequency subset comprises reducing the error count for ablocked frequency subset after each long-term interval for a definednumber of long-term intervals.
 18. The method of claim 11, wherein thereare twelve frequency subsets of which ten are active and two areblocked.
 19. The method of claim 18, wherein the twelve frequencysubsets each comprise eight frequencies from a frequency band dividedinto ninety-six frequencies.
 20. The method of claim 19, wherein thefrequency band is an ISM band from 2.4 GHz to 2.4835 GHz.
 21. The systemof claim 1, wherein a blocked frequency subset has previously been anactive frequency subset.
 22. The system of claim 1, wherein substitutinga blocked frequency subset for an active frequency subset comprisesusing both the blocked frequency subset and the active frequency subsetfor frequency hopping for a defined period before completing thesubstitution.
 23. The method of claim 11, wherein a blocked frequencysubset has previously been an active frequency subset.
 24. The method ofclaim 11, wherein substituting a blocked frequency subset for an activefrequency subset comprises using both the blocked frequency subset andthe active frequency subset for frequency hopping for a defined periodbefore completing the substitution.